Optical recording method, optical recording medium, and optical recording system

ABSTRACT

An optical recording method suited for recording a very small object or movement of a living organism such as a microorganism, which comprises irradiating an informative object set on or above a recording layer comprising a photosensitive material capable of undergoing a storable and detectable photochemical reaction, preferably a polymer material containing a photoreactive component capable of photoisomerization and having in the repeating unit thereof at least one group selected from a urethane group, a urea group, an amide group, a carboxyl group and a hydroxyl group, and recording a distribution of an optical near field generated from the informative object being irradiated on the photosensitive material as a photoreacting quantity of the photosensitive material.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to an optical recording method, an opticalrecording medium, an optical recording system, and a method forobserving a sample. More particularly, it relates to an opticalrecording method and an optical recording system making use of adistribution of an optical near field produced from an informativeobject being irradiated, an optical recording medium having excellentheat resistance, and application of the optical recording method tosample observation.

[0003] 2. Description of the Related Art

[0004] Methods for optically recording information on a recording mediumcomprising irradiating the recording medium with, for example, acondensed recording light beam from a laser light source to change thereflectance, etc. of the recording medium and recording the change areknown. However, not only a laser beam but any other optical systemsutilizing light transmitted through gas, etc. cannot be made use of inthe region below the diffraction limit of the light. Recording densitiessmaller than the scale of the diffraction limit can never be achieved,nor can be recorded information of an object smaller than thediffraction limit.

[0005] In recent years a so-called optical near field has beenattracting attention in this connection. An optical near field can belocalized in regions smaller than the wavelength of the light on thesurface of an object. Hence, application of an optical near field tohigh-density recording systems and high-resolution optical microscopeshas been proposed.

[0006] For example, Motoichi Ohtsu made a report on an optical nearfield microscope called “C mode” or “I mode” in his paper titled “ThePresent and Future Development of Optical Near Field Microscope” (Kikaino kenkyu, Vol. 49, No. 5 (1997)). The optical near field microscope ofC mode picks up evanescent light (optical near field) generated on thesurface of a sample being irradiated, by scanning with a fine probe toprovide optical data, which are processed to give a three-dimensionalimage of the surface of the sample. The optical near field microscope ofI mode uses a fine probe through which light is transmitted to ooze outan optical near field oozes from its tip. The surface of a sample isscanned with this fine probe to convert the optical near field toscattered light thereby furnishing information data of the samplesurface.

[0007] However, the above-described optical near field microscopes of Cmode and I mode involve the following disadvantages.

[0008] (1) Because the probe is brought close to a sample to beobserved, it greatly disturbs the electric field around the sample.Therefore, the resulting image is difficult to interpret.

[0009] (2) Because a scatter type probe having a very small opening or avery small diameter at the tip is used, the detectable light intensityis small, and the signal/noise ratio (S/N ratio) is not sufficient.

[0010] (3) Such processing as integration is necessary for improving theS/N ratio. Considering that scanning with the probe needs some time, themicroscope meets difficulty in making an observation on high-speedphenomena or biological cells.

[0011] As to optical recording media used in various optical recordingmethods, it has been keenly demanded to develop a recording mediumhaving high record durability (especially heat resistance) and/or havingrecorded practically advantageous information.

[0012] S. Davy and M. Spajer report in their paper “Near Field Optics:Snapshot of the field emitted by a nanosource using a photosensitivepolymer” (Appl. Phys. Lett., Vol. 69, No. 22, p. 3306 (1996)) atechnique comprising applying an optical near field generated from thetip of a probe to a photosensitive polymer film of an acrylic polymerhaving an azo dye in the side chain thereof to produce unevenness, whichis not optical information recording. This technique is to record theoptical near field of a light source. A method for recording the opticalnear field of an informative object is not disclosed in their report.Nor is given consideration to the thermal stability of the record.

[0013] JP-A-61-287791 discloses an optical recording medium making useof a condensational polymer dye, which is characterized by inertness tophoto-induced chemical degradation or change of optical properties.

SUMMARY OF THE INVENTION

[0014] A first object of the present invention is to provide an opticalrecording method, an optical recording system, an optical recordingmedium, and a method for observing a sample which are free from theabove-described disadvantages (1) to (3) and to provide a technique ofoptical near field memory for achieving an ultrahigh recording densityof several tens of gigabites per square inch and a photolithographictechnique applicable to the region below the light diffraction limit.

[0015] A second object of the present invention is to provide an opticalrecording medium having excellent record durability, particularly heatresistance.

[0016] The inventors of the present invention have found that, when aninformative object (a sample for observation or an object for puttinginformation in) is positioned on or above the surface of aphotosensitive material, and that area of the photosensitive material isirradiated with light, photochemical reaction of the photosensitivematerial takes place more strongly with the optical near field at thepart where the irradiated informative object is positioned than with theirradiating light at other irradiated parts. The first object of theinvention is accomplished based on this finding.

[0017] The inventors have also found that the second object of theinvention is accomplished by an optical recording medium prepared byusing a polymer containing a photoreactive component capable ofphotoisomerization and having in the repeating unit thereof at least onegroup selected from a urethane group, a urea group, an amide group, acarboxyl group and a hydroxyl group. Based on this finding, there areprovided a recording medium capable of recording optical informationfurnished from light for irradiation or an optical near field in avariety of modes, a recording medium useful for holography, a recordingmedium which contains a specific photoreactive component capable ofrecording, reading out and erasing information and providing a durableand heat-resistant record, and a recording medium having effectivelyrecorded thereon changes of an informative object with time.

[0018] The first object of the invention is accomplished by thefollowing 1st to 4th aspects, and the second object of the invention isachieved by the following 5th to 8th aspects.

[0019] The 1st aspect of the invention is an optical recording methodcomprising constituting a recording layer of a photosensitive materialcapable of undergoing a storable and detectable photochemical reaction,setting an informative object on or above the recording layer at such aposition that an optical near field generated from the informativeobject may reach the recording layer, irradiating at least the area ofthe recording layer where the informative object is positioned withlight to cause the informative object to generate the optical nearfield, and recording the distribution of the optical near field on thephotosensitive material as a photoreacting quantity of thephotosensitive material.

[0020] The 2nd aspect is an optical recording method comprisingconstituting a recording layer of a photosensitive material capable ofundergoing a storable and detectable photochemical reaction, setting amobile or moving informative object on or above the recording layer,irradiating at least the area of the recording layer where theinformative object is positioned with light to cause the informativeobject to generate the optical near field, and recording thedistribution of the optical near field on the photosensitive material asa photoreacting quantity of the photosensitive material, the irradiationand recording being repeated two or more times in accordance with themovement of the informative object.

[0021] The 3rd aspect provides an optical recording system comprising arecording layer on or above which an informative object is positionedand which is constituted by a photosensitive material capable ofundergoing a storable and detectable photochemical reaction, and a lightsource capable of irradiating at a time at least the area of therecording layer where the informative object is positioned.

[0022] The 4th aspect provides a method for observing a samplecomprising a recording process comprising constituting a recording layerwith a photosensitive material capable of undergoing a storable anddetectable photochemical reaction, setting a sample to be observed on orabove the recording layer at such a position that the optical near fieldgenerated from the sample being irradiated may reach the recordinglayer, irradiating at least the area of the recording layer where thesample is positioned to cause the sample to generate an optical nearfield, and recording the distribution of the optical near field on thephotosensitive material as information corresponding to thephotoreacting quantity of the photosensitive material, and an observingprocess comprising observing the recorded information by an observingmeans selected according to the recording mode.

[0023] The 5th aspect resides in an optical recording medium comprisinga recording layer for recording optical information with light forirradiation or a generated optical near field, wherein the recordinglayer comprises a polymer material containing a photoreactive componentcapable of photoisomerization and having in the repeating unit thereofat least one group selected from the group consisting of a urethanegroup, a urea group, an amide group, a carboxyl group and a hydroxylgroup.

[0024] The 6th aspect is an optical recording medium for holography,which has a recording layer comprising a polymer material containing aphotoreactive component capable of photoisomerization and having in therepeating unit thereof at least one group selected from the groupconsisting of a urethane group, a urea group, an amide group, a carboxylgroup and a hydroxyl group.

[0025] The 7th aspect furnishes an optical recording medium capable ofrecording, reading out and erasing information which has a recordinglayer comprising a polymer material containing a photoreactive componentwhich is capable of photoisomerization and the molecular orientation ofwhich can be controlled by light and having in the repeating unitthereof at least one group selected from the group consisting of aurethane group, a urea group, an amide group, a carboxyl group and ahydroxyl group.

[0026] The 8th aspect affords an optical recording medium having arecording layer comprising a photosensitive material capable ofundergoing a storable and detectable photochemical reaction, therecording layer having recorded thereon at least one of the followingpieces of information (1) to (4) in such a mode that a distribution ofan optical near field generated from an informative object beingirradiated is recorded:

[0027] (1) a record of an instantaneous form of a moving informativeobject;

[0028] (2) a record of movement of an informative object which is a fineparticle movable by radiant pressure of light;

[0029] (3) a record of movement of an informative object which is anautonomically moving living organism; and

[0030] (4) a record of changing history of an informative object whichshows change with time that can be recorded as optical information.

BRIEF DESCRIPTION OF THE DRAWINGS

[0031]FIGS. 1 through 6 are micrographs obtained under an atomic forcemicroscope in Examples.

[0032]FIG. 7 is a graph of depth of recorded depressions vs. irradiatinglight intensity.

[0033]FIG. 8 shows the optical system used in Examples.

[0034]FIG. 9 is a micrograph obtained under an atomic force microscopein Example.

[0035]FIG. 10 shows the results of measuring the shape of a grating inExample.

[0036]FIG. 11 shows another optical system used in Example.

[0037]FIG. 12A, 12B, and 12C show the state of recording, erasure, andre-recording of information, respectively.

DETAILED DESCRIPTION OF THE INVENTION

[0038] [Optical Recording]

[0039] The term “optical recording” as used herein has a broader sensethan the term “information recording” as commonly intended. That is, itnot only means recording for storing the information from an informativeobject having known information converted but includes the case in whichan informative object is per se an object of analysis as in optical nearfield microscopic analysis, and its analytical data are recorded. Theterm is also applied to the case in which an informative object is usedas information and a means for fine photoprocessing of a photosensitivematerial as in semiconductor lithography.

[0040] The 1st to 3rd aspects of the invention will be described indetail.

[0041] [Photosensitive Material]

[0042] Any photosensitive material capable of undergoing an arbitrarystorable and detectable photochemical reaction can be used in theinvention with no particular limitation. For example, materials whichundergo photoisomerization, etc. in accordance with light intensity toproduce unevenness on its surface in agreement with the reactionquantity. Polymer materials are particularly preferred.Photoisomerization is a preferred photochemical reaction for itsrapidness in response to light.

[0043] Where unevenness is formed on the surface of the photosensitivematerial, there is an advantage that information of an informativeobject is recorded as a physically fixed shape that can be observed witha means having overwhelmingly higher spacial resolution than opticalmicroscopes, such as an atomic force microscope (AFM), a scanningtunnelling microscope (STM), a scanning electron microscope (SEM), andthe like.

[0044] Photosensitive materials showing changes in refractive index orabsorbance in accordance with light intensity are also useful. In thiscase, too, the information recorded as a refractive index or absorbancedistribution can be observed or detected by an appropriate known meanssuch as an optical near field microscope.

[0045] While polymeric photosensitive materials are generally preferred,other photosensitive materials are also employable. Of polymermaterials, condensational polymer materials such as polyester,polyamide, polyurethane, and polyurea are particularly preferred fortheir high capacity of introducing photoreactive sites forphotoisomerization, etc.

[0046] [Recording Layer]

[0047] The recording layer is not particularly limited in shape as longas a sample to be observed can be positioned on or above it, while aflat surface made of a film of the photosensitive material is generallyconvenient for optical recording and for observation and/or detection ofthe recorded information. In carrying out optical recording forphotoprocessing, an arbitrary surface of the object of photoprocessingserves as a recording layer. The recording layer can have an arbitraryarea according to necessity.

[0048] The recording layer is usually placed in the atmosphere but, ifdesired, under pressure or reduced pressure. In observingmicroorganisms, etc., the recording layer can be covered with a waterdroplet or, in some cases, the essential part or the whole of the systemmay be immersed in liquid, e.g., water.

[0049] [Informative Object]

[0050] The term “informative object” as intended in the inventionincludes various embodiments, such as an object having converted knowninformation to be optically recorded, an object to be analyzed, aprocessing means for carry out fine photoprocessing and the like.

[0051] The informative object is not essentially limited in shape, sizeor material as long as it generates an optical near field on beingirradiated. When used for optical information recording orphotoprocessing, the informative object preferably has a controlledshape, controlled transparency or a controlled refractive index from thestandpoint of accuracy. Where only one side of an informative object(for example, the side opposite to the recording layer) is irradiated,it is preferred for the informative object to have light transmittingproperties above a certain level or to have a small size enough togenerate an optical near field on its side in contact with or facing tothe recording layer.

[0052] The size of the informative object may be either above or belowthe diffraction limit of irradiating light. Where it is used for highdensity optical information recording, it is desirable that a singlerecording bit be formed within a range equal to or smaller than thediffraction limit. More specifically, the informative object ispreferably 100 nm or smaller in size to realize a recording density ashigh as several tens of gigabites per square inch (Gbit/in²) or evenhigher. A still preferred size is 25 nm or smaller for achieving arecording density of 1 Tbit/in². While not limiting, the material of theinformative object preferably includes transparent glass and polymersfor controllability and handling properties.

[0053] The informative object to be irradiated is preferably set at aposition within a several hundreds of nanometers' distance from therecording layer so that an optical near field generated from theinformative object being irradiated may reach the recording surface. Astill preferred distance from the recording layer is within 100 nm sothat a sufficient optical near field generated from the informativeobject may reach the recording layer to realize accurate recording. Itis particularly preferred for the informative object to be positioned indirect contact with the recording layer when irradiated. In this case,the optical near field sufficiently and certainly reaches the recordinglayer to further enhance the accuracy of recording.

[0054] [Irradiating Light]

[0055] The wavelength of irradiating light is not particularly limitedand can be selected appropriately in conformity with the photosensitivematerial making up of the recording layer. Seeing that an optical nearfield generated from an irradiated informative object is absorbed by thephotosensitive material to cause a prescribed photochemical reaction,wavelengths showing a high absorption efficiency are preferablyselected. In general, rays from the ultraviolet to near infrared regionare chosen.

[0056] The light source for irradiation is not particularly limited andcan be selected appropriately according to the optical near field to berecorded. In view of reproducibility in forming unevenness as arecording mode or ease in the analysis following, a laser light sourceis preferred.

[0057] The intensity of light or time of irradiation is not limited andis selected appropriately in accordance with the photoreactivity, andthe like of the photosensitive material. In recording high-speedmovement of an informative object by repeating short time exposure,pulse light having a high peak power can be used.

[0058] With respect to the range to be irradiated, the language “thearea where an informative object is positioned” as used herein denotesthe range of the surface area of the recording layer containing theinformative object. A requisite or useful area range is decidedarbitrarily in accordance with the purpose of the optical recording.

[0059] [Distribution of Optical Near Field]

[0060] The language “distribution of an optical near field generatedfrom an informative object being irradiated” used as for the 1st aspectof the invention chiefly means pieces of information relating to theshape and position of an informative object. The same language used asto the 2nd aspect of the invention additionally means pieces ofinformation relating to a trace or form of movement of an informativeobject or change of an informative object in characteristics with time.

[0061] The 1st aspect of the invention produces the following actionsand effects.

[0062] When the area of the recording layer containing an informativeobject is irradiated, the photochemical reaction induced by the actionof the optical near field in the area where the informative object ispositioned is stronger than the photochemical reaction induced by theirradiating light on the other area. As a result, the distribution ofthe optical near field generated from the informative object isreflected as a difference in photochemically reaction quantity of thephotosensitive material between the irradiated area and non-irradiatedarea. The mechanism of such a phenomenon could be attributed to variouscauses and has not been proved definitely as yet. For example, thehigher refractive index of an informative object than that of the lighttransmitting medium (e.g., air) may have some influences, or an opticalnear field may have such properties that induce a particularly strongphotochemical reaction.

[0063] According to the present invention, there is no need to use aprobe for picking up only the optical near field of an informativeobject as in the above-mentioned conventional C mode nor a probe forcausing an optical near field to act only on an informative object as inthe conventional I mode. Therefore, recording completes simply byirradiating once without requiring the time for scanning with a probe.The above-described other disadvantages accompanying the use of a probeare also eliminated.

[0064] Since optical recording completes through a single irradiatingoperation, there is no such a recording failure that may occur where aninformative object is a very small substance or a living organism(especially a microorganism) which moves or varies its refractive indexdistribution with time. Because the whole of the predetermined area isirradiated at a time, it is possible to simultaneously record a varietyof informative objects positioned randomly in that area, tosimultaneously record a number of informative objects that arepositioned with some informative intention, or to record all pieces ofinformation of a large informative object at a time.

[0065] The optical record obtained by the present invention can besubjected to observation and/or detection by making use of an arbitraryand advantageous means for observing and/or detecting in conformity withthe photochemical reaction type either immediately after recording oranytime after storage.

[0066] Taking advantage of an optical near field, the optical recordingmethod of the invention can be applied to high-density opticalinformation recording, high resolution optical analysis or finephotoprocessing in a region equal to or smaller than the lightdiffraction limit.

[0067] The 2nd aspect of the invention produces the following actionsand effects in addition to those described as to the 1st aspect.

[0068] The movability of an informative object, such as a fine particlemovable by radiation pressure of light or an autonomically movingmicroorganism, rather enhances the merit of the 2nd aspect. That is,irradiation being repeated to cope with the movement of such aninformative object, not only the static information, such as the shape,but the movement or change in shape or properties of the informativeobject can be recorded.

[0069] For example, the method is effective in optically recording aphenomenon that a very small informative object is aligned along theelectric field distribution on receiving the radiation pressure oflight. This phenomenon can be made use of in optically recording thetrace of movement of the informative object while controlling themovement, or in performing photoprocessing at the moving site. Where aninformative object is accompanied by change in properties or form, themethod can be used to obtain an image showing the change with time.Further, the moving state or cell division or conjugation of amicroorganism can be traced.

[0070] In carrying out the above-described recording, short pulse lightcan be used to record a high-speed phenomenon continuously so that thephenomenon may be observed later slowly.

[0071] The 3rd aspect of the present invention has the following actionsand effects.

[0072] The optical recording system of the 3rd aspect makes it feasibleto effectively carry out the optical recording according to the 1stand/or 2nd aspects. Requiring neither a probe nor a probe-relateddrive/control/optical system, the optical recording system enjoysconsiderable simplification and reduction in cost.

[0073] The 4th aspect of the invention will be described in detail.

[0074] [Photosensitive Material]

[0075] Any photosensitive material capable of undergoing an arbitrarystorable and detectable photochemical reaction can be used with noparticular limitation. For example, photoreactive polymers are used forformation of unevenness on the recording layer. Photoconductivematerials are used for developing an electric potential difference onthe recording layer. Photorefractive materials are used for making achange in refractive index.

[0076] Photoreactive polymer materials are polymers having aphotoreactive site for absorbing light and thereby undergoing areaction. Polymers capable of photoisomerization, etc. depending on thelight intensity to product unevenness on its surface in accordance withthe reaction quantity are preferred. Photoisomerization is a preferredphotochemical reaction for its rapidness in response to light. Thesepolymer materials are also effective in producing recordable changes ofoptical characteristics, such as a refractive index and an absorbance.Condensational polymers, such as polyester, polyamide, polyurea, andpolyurethane, are preferably used for their high capacity of introducingphotoreactive sites.

[0077] The photoconductive materials, which may be either organic orinorganic, preferably include bismuth silicon oxide, and a polymer blendor copolymer comprising an electroconductive polymer.

[0078] The photorefractive materials, which may be either organic orinorganic, preferably include lithium niobate and a polymer blend orcopolymer comprising an electroconductive material and a nonlinearoptical material.

[0079] [Recording Layer]

[0080] The recording layer is not particularly limited in shape as longas a sample to be observed can be positioned on or above it, while aflat surface made of a film of the photosensitive material is generallyconvenient for the recording process and the following observingprocess.

[0081] The recording layer is usually placed in the atmosphere in therecording process but, if desired, under pressure or reduced pressure.In observing microorganisms, etc., the recording layer can be coveredwith a water droplet or, in some cases, the essential part or the wholeof the system may be immersed in liquid, e.g., water, or a specific gas.

[0082] [Sample to be Observed]

[0083] The sample to be observed is usually a very small object or amicroorganism but is not essentially limited in shape, size or materialas long as it generates an optical near field on being irradiated. Whereonly one side of a sample (for example, the side opposite to therecording layer) is irradiated, it is preferred for the sample to havelight transmitting properties above a certain level or to have a smallsize enough to generate an optical near field on its side in contactwith or facing to the recording layer.

[0084] The size of the sample may be either above or below thediffraction limit of light for irradiation. The advantages of the 4thaspect of the invention are manifested particularly effectively whenapplied to samples having a size equal to or smaller than thediffraction limit.

[0085] [Irradiating Light]

[0086] The wavelength of light used for irradiation is not particularlylimited and can be selected appropriately in conformity with thephotosensitive material making up the recording layer and a sample to beobserved. Seeing that an optical near field generated from a samplebeing irradiated is absorbed by the photosensitive material to cause aprescribed photochemical reaction, wavelengths showing a high absorptionefficiency are preferably selected. In general, rays from theultraviolet to near infrared region are chosen.

[0087] The light source for irradiation is not particularly limited andcan be selected appropriately according to the optical near field to berecorded. In view of reproducibility in unevenness formation as one ofrecording modes or ease in the following analysis, a laser light sourceis preferred.

[0088] The intensity of light and the time of irradiation are notlimited, either. They are decided appropriately in accordance with thephotoreactivity, and the like of the photosensitive material. Inrecording high-speed movement of a sample by repeating short timeexposure, pulse light having a high peak power can be used.

[0089] [Recording of Moving Sample]

[0090] The observing method of the 4th aspect is applicable toobservation of a moving sample or a sample being moved. It is knownthat, when light rays illuminate a very small sample of, for example,several nanometers to several tens of nanometers to show reflection orrefraction, the momentum of photons changes to exert force on thesample. It follows that the sample under observation is pushed or pulledby the radiation pressure of light. Accordingly, it is possible to makea small sample, e.g., microbial cells, move by controlling the lightintensity thereby to observe the sample on another predetermined site ofthe recording layer. This is effectively applicable to an embodiment inwhich a sample whose shape or recordable properties change with time issuccessively recorded with time while being made to move by irradiationwith pulse light, an embodiment in which two samples are made to move tothe same site of observation where they undergo reaction with eachother, and the reaction result is recorded, or an embodiment in whichmovement, cell division or cell conjugation of a microorganism istraced.

[0091] The wavelength of the light for controlling the movement of avery small sample and that of the light for furnishing image informationmay be changed to improve the accuracy of recording and observation.

[0092] In continuous recording, every individual image obtained is arecord resulting from instantaneous irradiation. Therefore the imageobtained is free from blur or recording failure that may be caused bythe change of the sample in position or properties.

[0093] [Observing Process]

[0094] Where the recording process is to form unevenness on therecording layer of the photosensitive material, there is an advantagethat information of a sample is recorded as a physically fixed shapethat can be observed with a means having overwhelmingly higher spacialresolution than optical microscopes, such as AFM, STM, SEM, TransmissionElectron Microscope (TEM), Scanning Frictional Force Microscope, and thelike.

[0095] The information recorded as a refractive index distribution or anabsorbance distribution can be read out and/or observed by means of ascanning near-field optical microscope, and the information recorded asa change in surface electric potential of the recording layer can beread out and/or observed by means of a surface electric potentialmicroscope (e.g., scanning Maxwell stress microscope or a scanningKelvin probe force microscope), and so forth.

[0096] The 4th aspect of the invention produces the following actionsand effects.

[0097] In the recording process (A), the area of the recording layercontaining a sample is irradiated with light whereby a photochemicalreaction takes place in the area where the sample is positioned by theaction of the optical near field generated by the sample, and thisreaction is stronger than the photochemical reaction which takes placein the other irradiated area. As a result, the distribution of theoptical near field generated from the sample is recorded on a leveldifferent from the level of the surrounding area as informationcorresponding to the photoreacting quantity of the photosensitivematerial.

[0098] The mechanism of such a phenomenon could be attributed to variouscauses and has not been proved definitely as yet. For example, thehigher refractive index of a sample than that of the light transmittingmedium (e.g., air) may have some influences, or an optical near fieldmay have such properties that induce a particularly strong photochemicalreaction.

[0099] According to the recording process (A), there is no need to use aprobe for picking up only the optical near field of a sample as in theabove-mentioned conventional C mode nor a probe for causing an opticalnear field to act only on a sample as in the conventional I mode.Therefore, recording completes simply by irradiating once withoutrequiring the time for scanning with a probe. The above-described otherdisadvantages accompanying the use of a probe are also eliminated.

[0100] Since the recording process (A) takes advantage of an opticalnear field, it achieves a resolving power corresponding to the lightdiffraction limit or even finer. Further, exposure with light completesthrough a single irradiating operation. Therefore, there is no such arecording failure that may occur where a sample is a very smallsubstance or microorganism which moves autonomically or be moved by theradiation pressure of light. It has now been made feasible to make anobservation of a sample that is moving at a high speed with a higherresolving power than the light diffraction limit. By repeating shorttime exposure with pulse light, a moving sample can be observed as aseries of still images.

[0101] Moreover, a predetermined area of the recording layer isirradiated with light at a time, all the variety of, or a large numberof, objects present in that area can be recorded simultaneously.

[0102] The optical record obtained by the recording process (A) isobserved or read out in the observing process (B) either immediatelyafter recording or anytime after recording. The means for observing orreading out the optical record is selected appropriately according tothe type of the photochemical reaction (i.e., the mode of opticalrecording).

[0103] Because the object of observation is fixed on the recording layerof the photosensitive material as optically recorded information, thedisadvantage due to unexpected movement of the object is eliminated.Even where the means of observation have many restrictions on use, forexample, a scanning tunneling microscope or a scanning atomic forcemicroscope, observation or reading out can be carried out making use ofthe merits of these means without being hindered by the restrictions.

[0104] The 5th aspect of the invention will now be explained in detail.

[0105] [Optical Recording Medium]

[0106] The optical recording medium of the 5th aspect may comprise, inaddition to the recording layer comprising the above-described polymermaterial, other constituent members, such as a substrate, a protectivefilm, and a reflective film.

[0107] The optical recording medium can be used as a medium forrecording optical information in various known applications, forexample, a recording medium for calculators, an audio-visual recordingmedium, a recording medium for recording an optical near field intensitydistribution.

[0108] For example, the optical recording medium can be used as a mediumfor writing only once like CD-R, on which information is recorded in theform of unevenness, etc. for every bit by use of an optical near fieldmicroscope or all at once by irradiation of light through a mask toprovide read-only memory (ROM). Information of an object to be observedcan be recorded by putting the object on the recording medium,irradiating the object, and recording the intensity distribution of thethus generated optical near field on the recording medium in the form ofunevenness, etc.

[0109] The information recorded as unevenness can be read with an atomicforce microscope, a stylus-type profiler, a laser displacement meter,etc. The information recorded as a refractive index distribution can beread with a phase-contrast microscope, an optical near field microscope,etc. The information recorded as a difference of orientation of thephotoreactive component (hereinafter described) can be read from apolarized visible or infrared light absorption spectrum, etc.

[0110] [Recording Mode and Structure]

[0111] The information recorded on the optical recording medium can havevarious modes of recording based on the photoisomerization reaction ofthe photoreactive component. Such recording modes include change ordifference occurring on the surface of the optical recording medium(i.e., surface recording layer), such as unevenness, a change inrefractive index or refractive index anisotropy, a change in absorbanceor absorbance anisotropy, a difference in degree of orientation of thephotoreactive component, and theses changes or differences in opticalcharacteristics which occur in a recording layer or layers providedinside the optical recording material.

[0112] A change in refractive index or absorbance or a difference indegree of orientation of the photoreactive component can also be inducedby using irradiating light transmitted through air, etc. and recorded onthe surface of the optical recording medium or the recording layer orlayers provided inside the recording medium.

[0113] While the light used for recording and detecting information,i.e., writing and reading information is not particularly limited,ultraviolet light, visible light or near infrared light are preferablyused in practice.

[0114] The optical recording medium of the invention can have a singlerecording layer or a plurality of recording layers, sometimes 100 ormore recording layers. In the former case, information is recorded onthe surface of the medium (surface recording layer) or on a singlerecording layer which is provided inside of the medium for the purposeof protecting the record. Where information is recorded on the surfacerecording layer, the recorded side or both sides of the medium can beprotected by coating with a protective film.

[0115] In the latter case, information is recorded on the surfacerecording layer and one or more recording layers provided inside themedium, or information is recorded on two or more recording layersprovided inside the medium. A plurality of thin optical recording mediumunits arbitrarily selected from those having a single recording layer(i.e., the surface recording layer or the inside recording layer) andthose having two or more recording layers can be joined to provide anoptical recording medium having a plurality of recording layers.

[0116] Where the optical recording medium has two or more recordinglayers, the recording modes do not need to be the same in all therecording layers. For example, information may be recorded on thesurface recording layer as unevenness by making use of an optical nearfield, on an inside recording layer as a refractive index distribution,and on another inside recording layer as an absorbance distribution. Ifdesired, a buffer layer which does not participate in recording may beprovided between adjacent recording layers for the purpose of reducingcross talks between them.

[0117] [Polymer Material]

[0118] The polymer material which can be used in the optical recordingmedium of the invention is not limited except that it is a polymercontaining a photoreactive component capable of photoisomerization andhaving in the repeating unit thereof at least one group selected fromthe group consisting of a urethane group (—O—CO—NH—), a urea group(—NH—CO—NH—, —NH—CO—N═ or —NH—CO—N<), an amide group (—CO—NH—), acarboxyl group and a hydroxyl group.

[0119] The degree of polymerization of the polymer is not particularlylimited as far as is consistent with moldability, e.g., film formingproperties. The polymer may be either a homopolymer or a copolymer andcan have an arbitrary molecular structure, such as a linear structure, abranched structure, a ladder structure, a star-burst structure, etc. Theform of the copolymer includes a block copolymer, a random copolymer, agraft copolymer, and so forth. When particularly improved heatresistance is expected, a polymer having a ring structure, e.g., aphenylene group, in its main chain is preferred.

[0120] The language “containing a photoreactive component” as used asfor the polymer material constituting the optical recording medium ofthe invention means that a photoreactive component is bonded to thepolymer through a chemical bond, such as a covalent bond, an ionic bondor a coordinate bond, as hereinafter described in detail. Such aphotoreactive component preferably includes those having at least one ofan azo group, a C═C group and a C═N group which are capable of trans-cisphotoisomerization.

[0121] On being irradiated with light having a usual intensity, thephotoreactive component capable of trans-cis photoisomerization changesits trans to cis configurational ratio. Because the trans-form and thecis-form differ in optical characteristics such as refractive index andabsorbance, the difference in the configurational ratio between theirradiated area and non-irradiated area makes recording possible. Whenirradiated with high intensity light, on the other hand, the double bondof N═N, C═C or C═N is reacted to produce low molecular weight segments.The low molecular weight segments evaporate off to cause a reduction indensity or a change in shape (e.g., formation of a depression of thepolymer material), which makes recording possible.

[0122] Further, a so-called optical poling effect on the photoreactivecomponent capable of trans-cis photoisomerization can be taken advantageof. The photoreactive component capable of trans-cis photoisomerizationis generally anisotropic as to light absorption. For example, when apolymer having a trans-4-amino-4′-nitroazobenzene structure as aphotoreactive component is irradiated with light of 488 nm that ispolarized in the direction parallel to the direction connecting theamino group and the nitro group, the above structure undergoesphotoisomerization into a cis-configuration at a higher probability thanwith polarized light equal in wavelength or intensity but different indirection of polarization. Next, the cis-configuration changes into twokinds of trans-configuration by light or heat. One (A) shows thedirection connecting the amino group and the nitro group almost parallelto the direction of polarized incident light, and the other (B) showsthe direction connecting the amino group and the nitro group almostperpendicular to the direction of polarized incident light. In thisregard, although the probabilities of producing the trans-configurations(A) and (B) are almost the same, that of absorbing thetrans-configuration is different depending on anisotropy of absorbance.As a result, the above (A) to (B) ratio of the photoreaction componentdecreases to change the orientation distribution, and information canthus be recorded by making use of the difference of orientationdistribution between the irradiated area and non-irradiated area.

[0123] Taking the above observations into consideration, examples ofparticularly preferred polymers containing a photoreactive componentinclude the polymers used in Examples hereinafter given and, inaddition, those having a structure represented by formula (I) to (IV)shown below. In these formulae, —X represents a nitro group, a cyanogroup, a trifluoromethyl group, an aldehyde group or a carboxyl group;—Y— represents —N═N—, —CH═N— or —CH═CH—; and —R—, —R¹—, —R²— and —R³—,which may be the same or different, each represents a phenylene group,an oligomethylene group, a polymethylene group or a cyclohexylene group.

[0124] Additionally the polymer materials described in Examples orComparative Examples of the following publications are all employable inthe present invention as a polymer material containing a photoreactivecomponent: all the urethane copolymers and polyurethane described inExamples and Comparative Examples of JP-A-8-160477; all theurethane-urea copolymers and polyurethane described in Examples andComparative Examples of JP-A-8-220575; the polyurethane described inExample 1 and Comparative Example 1 of JP-A-8-87040; the ester-amidecopolymers described in Examples 1 and 3 of JP-A-10-90739; and thepolyamide described in Examples 3, 4 and 5 of JP-A-9-334794.

[0125] It is desirable that larger content of the photoreactivecomponent be present in the polymer material. Chemically bonded to thepolymeric structure, the photoreactive component could be present in ahigh proportion in a uniformly dispersed state in the polymer withoutsuffering from aggregation. A preferred content of the photoreactivecomponent in the polymer is from 30 to 70% by weight. If it exceeds 70%by weight, the monomer(s) tend(s) to fail to achieve a degree ofpolymerization sufficient for stable formation of a recording layer.

[0126] The 5th aspect produces the following actions and effects.

[0127] It is known that a urethane group, a urea group, an amide group,a carboxyl group and a hydroxyl group each form a strong hydrogen bondamong a kind or different kinds. Existence of at least one of, or atleast one kind of, these groups per repeating unit inducesintramolecular or intermolecular hydrogen bonding to form a structurelike a crosslinked structure. This structure serves to raise the glasstransition point of the polymer and thereby to improve the heatresistance of the polymer material, leading to improved thermalstability of the record. Besides, this structure is resistant againstdeformation or molecular movement even at the glass transitiontemperature or higher temperatures.

[0128] Since the polymer material constituting the optical recordingmedium contains a photoreactive component capable of photoisomerization,irradiation of the optical recording medium with irradiating light or anoptical near field results in a difference in the proportion of theisomers between the irradiated area and the non-irradiated area.According to this difference, optical information is recorded asunevenness or differences in optical characteristics, such as adifference in refractive index or absorbance.

[0129] The photoreactive component is chemically bonded to the polymerthrough a covalent bond, an ionic bond, a coordinate bond, etc. If thephotoreactive component is merely dispersed in a polymer mechanically,the maximum content of the photoreactive component that could bedispersed uniformly without aggregation is about 20% by weight. To thecontrary, the photoreactive component as chemically bonded to thepolymer molecule can exist uniformly in a higher proportion, e.g., about30 to 70% by weight, thus contributing to improvement of response tolight in recording.

[0130] The following is to describe the 6th aspect of the invention indetail.

[0131] [Optical Recording Medium]

[0132] The practice of the optical recording medium of the 6th aspect isbasically similar to that of the 5th aspect, except for its applicationto a phase hologram of surface relief type or volume type or to anamplitude hologram.

[0133] A phase hologram of surface relief type is an optical recordingmedium having recorded holographic information as unevenness on thesurface of its recording layer by photoisomerization reaction. A phasehologram of volume type is an optical recording medium having recordedholographic information as a change in refractive index in the inside ofits recording layer. An amplitude hologram is an optical recordingmedium having recorded holographic information in its recording layer asa change in amplitude of light caused by a change of transmittancethrough the recording layer.

[0134] The optical recording medium for holography may be a thin film,which can be used alone or together with a substrate. While thethickness of the thin film is not particularly limited, an about 5 μm orgreater thick film is usually used as a volume hologram for ensuringsufficient diffraction efficiency.

[0135] Laser light is usually used for recording holographic informationon the recording medium for holography. While not limiting, ultravioletlight, visible light and near infrared light are preferred for efficientrecording. The same preference applies to the light for reading therecorded holographic information.

[0136] Polymer materials constituting the recording layer of the opticalrecording medium, photoreactive components of the polymer materials,preferred embodiments in the practice, and preferred content of thephotoreactive component are the same as those described with respect tothe 5th aspect of the invention.

[0137] The 6th aspect of the invention offers the following actions andeffects.

[0138] The optical recording medium according to the 6th aspect can beused as a medium for holography, which is irradiated with object lightand reference light to produce an interference fringe to be recorded.More specifically, the property of forming unevenness on the surface ofthe recording layer can be applied to a phase hologram of surface relieftype; the property of producing a change in refractive index in therecording layer can be applied to a phase hologram of volume type; andthe property of making a change in transmittance can be applied to anamplitude hologram.

[0139] The optical recording medium for holography of the presentinvention is advantageous over the conventional holographic recordingmedia comprising inorganic photosensitive materials, such as gelatindichromate, in that recording can be accomplished in a dry process. Inaddition to this, the optical recording medium of the invention isadvantageous over the conventional holographic recording mediacomprising photopolymers in that a fixing operation is not necessary.

[0140] The thermal stability of the record owing to the characteristicsof the polymer material and the improved photoresponse sensitivity owingto the high content of the photoreactive component that have beendescribed with respect to the 5th aspect of the invention also apply tothe 6th aspect.

[0141] The details of the 7th aspect of the present invention aredescribed below.

[0142] [Optical Recording Medium]

[0143] The range of the photoreactive component used in the opticalrecording medium of the 7th aspect is limited as compared with that usedin the 5th aspect. That is, the polymer which can be used in the 7thaspect should have chemically bonded thereto a photoreactive component(1) which is capable of reversible photoisomerization and (2) whosemolecular orientation can be controlled by light.

[0144] In order for a photoreactive component to possess the abovecharacteristics (1) and (2), it is essentially required that thecomponent should have a moiety capable of reversible cis-transphotoisomerization, such as an N═N double bond or a C═C double bond, andthat the molecular axes of two trans-forms, with the double bond beingtaken as fixed, be in different directions. In connection to the latterrequirement, an N═N double bond satisfies the requirement whatevermolecular structure may be bonded to each side thereof, but a C═C doublebond fails to satisfy where it has symmetric molecular structures on itsboth sides.

[0145] With the above exception, the 7th aspect is the same with the 5thaspect in terms of the polymer materials constituting the recordinglayer of the optical recording medium, the kind of the photoreactivecomponents contained in the polymer materials, preferred embodiments inthe practice, and the preferred content of the photoreactive component.

[0146] Linearly polarized light can be used for recording information onthe optical recording medium. While the recording light is notparticularly limited in wavelength, a preferred wavelength is in thevicinity of the maximum absorption wavelength of the dye (photoreactivecomponent) which shows an optical poling effect, at which wavelength thedye exhibits high efficiency in re-orientation on photoisomerization.

[0147] Reading of recorded information on the optical recording mediumis conducted by using weak linearly polarized light that does notinfluence the record. The recorded information is detected or read outas a change of transmission based on the optical poling of the recordinglayer or as a change in reflected light intensity caused by the changein refractive index.

[0148] The direction of polarization of reading light is not limited asfar as the change in optical characteristics is detected. In principle,a direction perpendicular to the linearly polarized light used forrecording shows a greater degree of the change in opticalcharacteristics and is also preferred from the standpoint of sensitivityand S/N ratio.

[0149] The wavelength of the reading light is not limited, either. Itmay be the same or different from that of the recording light. A highsensitivity can generally be secured at wavelengths in the vicinity ofthe maximum absorption wavelength at which great changes in opticalcharacteristics are detected. Reading light having the same wavelengthas that of recording light produces an advantage that the cost as awhole optical system is reduced. Reading is also possible with whitelight.

[0150] Recorded information can be erased by using circularly polarizedlight, random polarized light or linearly polarized light having adifferent direction of polarization from that of recording light. Whenthe recording layer is irradiated with such polarized light, therecorded information is erased through the above-mentioned mechanism torestore, as a matter of course, the state that allows re-recording.Similarly to the recording light, it is preferred for efficient erasurethat the wavelength of erasing light to be in the vicinity of themaximum absorption wavelength of the dye (photoreactive component).

[0151] The 7th aspect has the following actions and effects.

[0152] Since the photoisomerization of the photoreactive component isreversible, and the molecular orientation of the photoreactive componentcan be controlled by light, the optical recording medium is capable ofrecording, reading out and erasing (to make the recording mediumre-writable) information. For example, a photoreactive component(photoresponsive dye) exhibiting large absorption anisotropy and capableof trans-cis photoisomerization undergoes optical poling (molecularorientation control) on being irradiated with linear polarized light.Accordingly, a polymer containing such a photoreactive component canrecord information with linear polarized light.

[0153] The recorded information can be detected or read out as a changein transmitted or reflected light intensity caused by a change intransmission or refractive index when irradiated with, as reading light,weak linearly polarized light that gives no influence on the record.

[0154] The recorded information can be erased by irradiation withcircularly polarized light or random polarized light to restore themolecular orientation resulting from optical poling to the originalrandom state or by irradiation with relatively intense linearlypolarized light having the direction perpendicular to the writing lightto change the optical characteristics in the perpendicular directionthereby restoring the optical characteristics in the direction parallelto the polarization to the original state.

[0155] Accordingly, information recording, reading out and erasure canbe performed with one light source. That is, a recording system can beset up simply by such operations as rotation, attachment or detachmentof a polarizing plate, which is advantageous for assembly operation andcost.

[0156] The thermal stability of the record owing to the characteristicsof the polymer material and the improved photoresponse sensitivity owingto the large content of the photoreactive component that have beendescribed with respect to the 5th aspect of the invention also apply tothe 7th aspect.

[0157] The 8th aspect of the present invention is then described.

[0158] [Optical Recording Medium]

[0159] The optical recording medium of the 8th aspect may comprise, inaddition to the recording layer comprising a polymer material, otherconstituent members, such as a substrate, a protective film, and a lightreflective film, as in the 5th aspect of the invention.

[0160] The optical recording medium is characterized by the contents andmode of the recorded information. The optical recording medium hasrecorded on its recording layer at least one of pieces of information(1) to (4) previously described. The following finding has made suchinformation recording feasible.

[0161] When an informative object (a sample for observation or an objectfor putting information in) is positioned on the surface of aphotosensitive material, and that area of the photosensitive material isirradiated, the photochemical reaction of the photosensitive materialwhich takes place by the optical near field at the part where theinformative object is positioned is stronger than the photochemicalreaction at other irradiated parts.

[0162] Based on this finding, the piece of information (1), i.e., aninstantaneous form of a moving informative object (e.g., a livingobject), is recorded by setting a mobile informative object on or abovethe recording layer of the photosensitive material, irradiating thatarea of the recording layer, and recording the distribution of theoptical near filed generated from the informative object beingirradiated as an optical reaction quantity of the photosensitivematerial. The pieces of information (2) to (4) can be recorded byrepeating the above operation two or more times on a moving informativeobject. The piece of information (4), i.e., a record of change with timeof an informative object, includes a record of an informative objectwhich is made to move and to react.

[0163] More specifically, a phenomenon that a very small informativeobject is aligned along the electric field distribution on receiving theradiation pressure of light and then optical near field is recorded.This phenomenon is made use of in optically recording the trace ofmovement of an informative object while controlling the movement. Wherean informative object is accompanied by change in properties or form,the image of the change with time can be obtained. Further, the movingstate, cell division or conjugation of a microorganism can be traced.

[0164] In carrying out the above-described recording, short pulse lightcan be used to record a high-speed phenomenon continuously so that thephenomenon may be observed later slowly. Since a single shot forobtaining individual images of continuous recording completes through asingle irradiating operation, there is no such a recording failure thatmay occur where an informative object is a very small moving substanceor living organism (especially a microorganism).

[0165] While not limiting, the polymer material used in the 5th aspectis particularly preferred for the photosensitive material constitutingthe recording medium of the 8th aspect. Additionally any knownphotosensitive polymer materials or non-polymer materials capable ofrecording an optical near field are employable. The information can berecorded in the similar modes as described for the 5th aspect, forexample in the form of unevenness, an change in refractive index, achange in absorbance, or any other known modes of recording. The meansfor reading the recorded information is arbitrary.

[0166] Utilizing an optical near field, the optical recording medium ofthe 8th aspect can have information recorded at a very high density.

[0167] The 8th aspect of the invention produces the following actionsand effects.

[0168] There is provided an optical recording medium having recordedpractically beneficial pieces of information, such as change or movementof an informative object, that could not be achieved by conventionaltechniques using an optical near field. The optical recording medium hasthus acquired an heightened value as a means for furnishing information.Relying on an optical near field, the optical recording medium hasrecorded information at a high recording density exceeding the limit oflight diffraction and thereby having a further heightened value.

[0169] The present invention will now be illustrated in greater detailby way of Preparation Examples and Examples, but it should be understoodthat the present invention is not deemed to be limited thereto. Unlessotherwise noted, all the percents are by weight.

PREPARATION EXAMPLE

[0170] Synthesis of Photoreactive Component:

[0171] In a mixture of 300 ml of water and 180 ml of a 36% hydrochloricacid aqueous solution was dissolved 30.43 g of 2-methyl-4-nitroaniline,and the solution was cooled to 3° C. To the solution was added asolution of 15.20 g of sodium nitrite in 100 ml of water, and theresulting solution was stirred at 3° C. for 1 hour. A solution of 39.05g of m-tolyldiethanolamine in a mixture of 300 ml of water and 30 ml ofa 36% hydrochloric acid aqueous solution was slowly added thereto over60 minutes, followed by stirring at 3° C. for 150 minutes to allow themixture to react.

[0172] The reaction mixture was neutralized with 141.6 g of potassiumhydroxide dissolved in 700 ml of water, and the crude product wascollected by filtration, washed with water, and dried. Recrystallizationwas repeated three times to give4-N,N-bis(2-hydroxyethyl)amino-2,2′-dimethyl-4′-nitroazobenzenerepresented by formula (V) having a melting point of 169° C. in a yieldof 62%.

[0173] Preparation of Photoreactive Component-containing Polymer I:

[0174] In 50 ml of N-methyl-2-pyrrolidone were dissolved 2.000 g of theabove prepared4-N,N-bis(2-hydroxyethyl)amino-2,2′-dimethyl-4′-nitroazobenzene and2.095 g of 4,4′-diphenylmethane diisocyanate and reacted by stirring atroom temperature for 15 minutes and then at 100° C. for 60 minutes. Thereaction mixture was cooled to 50° C., and a solution of 0.319 g oftrans-2,5-dimethylpiperazine in 20 ml of N-methyl-2-pyrrolidone wasadded thereto, followed by further reacting for 5 hours while stirring.The reaction mixture was heated to 115° C. under reduced pressure toevaporate 52 ml of N-methyl-2-pyrrolidone slowly over a 150 minuteperiod.

[0175] The resulting concentrate was diluted with 180 ml of pyridine andfiltered through a 0.1 μm filter. The filtrate was poured into ethanol,and the polymer thus precipitated was collected by filtration. Thepolymer was further purified by reprecipitation twice to give a polymerrepresented by formula (VI) (designated polymer I) in a yield of 92%.Glass transition temperature: 141° C. Intrinsic viscosity at 30° C. inN-methyl-2-pyrrolidone: 0.69 dl/g Maximum absorption wavelength: 475 nm

[0176]

[0177] Preparation of Photoreactive Component-containing Polymer II:

[0178] In a mixture of 5 ml of pyridine and 5 ml of1,1,2,2-tetrachloroethane were dissolved 0.700 g of the compound offormula (V) and 0.606 g of terephthalic acid chloride, and the mixturewas allowed to react at 130° C. for 2 hour while stirring. The reactionmixture was cooled to 30° C., and 0.285 g of1,3-bis(aminophenoxy)benzene was added thereto to allow the mixture tofurther react for3 hours with stirring. The resulting reaction mixturewas poured into ethanol, and the precipitate was collected byfiltration. The thus obtained polymer was dissolved inN-methyl-2-pyrrolidone, and the solution was poured into water. Theprecipitate was collected by filtration and dried under reduced pressureto give a polymer represented by formula (VII) (designated polymer II)in a yield of 31%. Glass transition temperature: 102° C. Intrinsicviscosity at 30° C. in N-methyl-2-pyrrolidone: 0.18 dl/g Absorptionmaximum wavelength: 480 nm

[0179]

EXAMPLE 1

[0180] Preparation of Recording Medium:

[0181] The polymer I represented by formula (VI) was dissolved inpyridine to prepare a 6.5% by weight polymer solution. After filtrationthrough a 0.2 μm filter, the polymer solution was spin coated on a slideglass at 1000 rpm and dried at 80° C. for 20 hours in vacuo to prepare athin film as a recording medium.

[0182] Observation of Optical Near Field Distribution in the Vicinity ofInformative Object:

[0183] A disk having a hole of 5 mm in diameter was cleaned byultrasonication and put on the recording medium. A few drops of waterhaving dispersed therein a large number of polystyrene microsphereshaving a diameter of 500 nm were dropped on the hole of the disk. Afterallowing the system to stand until water evaporated spontaneously, thearea of the recording medium where the polystyrene microspheres wereplaced (recording area) was irradiated with a laser beam having a beamdiameter of about 3 mm and a wavelength of 488 nm emitted from anair-cooled argon laser (output: 20 mW).

[0184] The recording medium was washed with water to remove part of thepolystyrene microspheres, and the irradiated area of the recordingmedium was observed under an atomic force microscope (SPI-3700,manufactured by Seiko Instruments Inc.). The micrographs of the sampletaken from different angles are shown in FIGS. 1 and 2. Each micrographshows microspheres 2 (informative object) remaining on the recordingmedium 1 and depressions 3 which correspond to the shape of themicrospheres having been removed.

EXAMPLE 2

[0185] The following experiment was carried out in order to confirm thatformation of depressions is by the optical near field generated from aninformative object. The same procedure as in Example 1 was repeated,except for using polystyrene microspheres having a diameter of 100 nm,which is about one-fifth of the wavelength of the recording light. Theresulting micrographs, taken from different angles, are shown in FIGS. 3and 4. Each micrograph shows depressions 5 corresponding to themicrospheres having been removed from the surface of the recordingmedium 4.

[0186] For reference, the same recording medium was irradiated with noinformative object put thereon and observed in the same manner as inExample 1 described above. As a matter of course, no unevenness wasobserved on the recording medium.

EXAMPLE 3

[0187] Preparation of Recording Medium:

[0188] The polymer I represented by formula (VII) was dissolved inpyridine to prepare a 6.5% polymer solution. After filtration through a0.2 μm filter, the polymer solution was spin coated on a slide glass at1000 rpm and dried in vacuo at 80° C. for 20 hours to prepare a thinfilm having a thickness of about 1 μm as a recording medium.

[0189] Bit Data Recording by Depression Forming:

[0190] The recording medium was irradiated with a condensed beam of anargon laser having a wavelength of 488 nm to record bit data. The bitdata were observed through an atomic force microscope (SPI-3700,manufactured by Seiko Instruments Inc.). The condenser used was aultra-long working distance objective lens manufactured by Mitsutoyo,having an numerical aperture of 0.55 and a magnification of 100. Thepower of irradiating light was about 200 μW, the beam diameter was about1 μm, and the exposure time was about 10 msec.

[0191] The micrographs of the bit data taken under the atomic forcemicroscope from different angles are shown in FIGS. 5 and 6. Eachmicrograph shows recorded bit data as depressions 5 at the irradiatedspots.

[0192] The recording medium 1 having the bit data was heated at 150° C.for 1 hour and then again observed under the atomic force microscope. Nodifference by heating was observed in the recorded data, provingexcellent heat resistance of the recording film.

[0193] Relationship Between Depth of Depressions and Intensity ofIrradiating Light:

[0194] The recording medium was irradiated with an argon laser beamhaving a wavelength of 488 nm and a varied intensity to formdepressions, and the relationship between the light intensity (W/cm²)and the depth of the depressions (nm) was examined. The results obtainedare shown in FIG. 7. It is seen from FIG. 7 that the depth of thedepressions is proportional to the light intensity of irradiation atleast in the measured range of light intensity.

[0195] Recording by Change in Absorbance:

[0196] The recording medium was irradiated with linearly polarized lightfrom an argon laser which had a wavelength of 514.5 nm, an intensity of1 W/cm², and whose direction of polarization was in parallel with therecording film for an exposure time of 10 minutes. After theirradiation, the absorbance of the recording film measured at awavelength of 500 nm showed a decrease by about 22%. When the recordingmedium was irradiated with laser light having an intensity of 2 W/cm²for 10 minutes in the same manner, the absorbance at 500 nm showed adecrease by about 58% against the non-irradiated recording medium. Ineither case, no depression were found formed in the irradiated areaunlike the above-described irradiation with argon laser light having awavelength of 488 nm.

[0197] It can be seen from these results that the irradiated area andthe non-irradiated area have different absorbances, indicatingfeasibility of recording information by making use of a difference inabsorbance. Such a change in absorbance seems attributable to thephotoisomerization of the azobenzene moiety, the photoreactive componentof the polymer I, from a trans-form to a cis-form.

[0198] Recording by Anisotropy of Absorbance:

[0199] The recording medium was irradiated with linearly polarized lightfrom an argon laser which had a wavelength of 514.5 nm, an intensity of1 W/cm², and whose direction of polarization was in parallel with therecording film for an exposure time of 10 minutes. The polarized lightabsorption spectrum of the recording medium was measured before andafter the irradiation. The spectrum before irradiation was isotropic inthe polarization direction parallel with the film, whereas that afterirradiation showed anisotropy. That is, in the irradiated recordingmedium, the absorbance (A2) in the polarization direction perpendicularto the polarization direction of the recording light and parallel withthe recording medium was higher than the absorbance (A1) in thepolarization direction parallel with the polarization direction of therecording layer.

[0200] This results show the difference in anisotropy of absorbancebetween the irradiated area and the non-irradiated area, provingfeasibility of recording information by making use of a difference inanisotropy of absorbance. Such a change in anisotropy of absorbanceseems ascribable to the photoisomerization and optical poling effect ofthe azobenzene moiety, the photoreactive component of polymer I.

[0201] Recording by Change in Refractive Index:

[0202] A recording medium was prepared in the same manner as describedabove, except that the coating layer was vacuum dried at 100° C. for 20hours and then at 150° C. for 10 hours. While being heated at 110° C.,the recording medium was irradiated with ultraviolet light from amercury lamp (USH-250BY, manufactured by Ushio Inc.) at an intensity of0.08 W/cm² for 4 hours. The UV-irradiated area showed a decrease inrefractive index at 830 nm by 0.003.

[0203] It is seen that the UV-irradiated area and non-irradiated areashow different refractive indices, which can be used for recordinginformation. When the recording medium having recorded the change inrefractive index in this manner was heated at 110° C. for 30 minutes,the refractive index was unchanged, proving the recording medium to beexcellent in thermal stability.

[0204] Recording by Change in Anisotropy of Refractive Index:

[0205] A recording medium was prepared in the same manner as describedabove, except that the coating layer was vacuum dried at 100° C. for 20hours. The resulting recording medium was heated at 110° C. for 4 hours,and the heat-induced change in anisotropy of refractive index wasmeasured. Taking the refractive index for light having a wavelength of830 nm and a polarization direction parallel to the recording layer asn1, while that for light having the same wavelength and a polarizationdirection perpendicular to the recording layer being taken as n2, therefractive index anisotropy, defined as n1−n2, was 0.022 before heating.On heating, it decreased to 0.016.

[0206] Heating a material locally by irradiation is sufficientlypossible with known techniques. The techniques can be applied to producea difference in refractive index anisotropy between an irradiated areaand a non-irradiated area, thereby making it feasible to recordinformation as a difference in refractive index anisotropy.

EXAMPLE 4

[0207] Holography:

[0208] Interference of light was recorded on the recording mediumprepared in Example 3 by using a two beam interference aligner shown inFIG. 8. In FIG. 8, a beam having a wavelength of 488 nm emitted from anargon laser light source 26 was reflected on a mirror 6 and passedthrough a pin hole 7 (diameter: 2 nm) for eliminating spatial noise ofthe laser beam to obtain a uniform interference pattern. The beam wasthen split into two beams by a beam splitter 8. The optical path andintensity ratios of the beams were made equal by means of a plurality ofmirrors 9 arranged in proper positions, and the two beams entered arecording medium 10. The grating space was adjusted according to theangle of incidence. The exposure was continued for 10 minutes.

[0209] The surface profile of the irradiated recording medium 10 wasobserved through an atomic force microscope. As shown in the micrographof FIG. 9, a recorded interference pattern of sine waves was observed.Examination on the relationship between irradiation energy and depth ofdepressions based on the results of observation revealed that the depthof depressions are proportional to the irradiation energy. This meansthat the intensity of interference light can be recorded as such on therecording medium 10. The recording medium 10 was thus proved to haveexcellent performance as a hologram. When the irradiated recordingmedium 10 was heat treated at 130° C. for 1 hour, no change in image wasobserved, proving excellent heat stability.

[0210] Applicability as Grating Coupler:

[0211] An interference pattern of third harmonic waves of an Nd:YAGlaser (wavelength: 355 nm) was recorded on the recording medium usingthe two beam interference aligner shown in FIG. 8. The results obtainedare shown in FIG. 12, in which a neat sine wave pattern having afrequency of 8 μm is recorded, proving that the recording medium hasexcellent performance as a hologram. The grating pattern prepared isapplicable as a grating coupler of a waveguide, etc. Experiments onoptical input characteristics revealed an input efficiency of 10% orhigher.

EXAMPLE 5

[0212] Recording, Reading Out, Erasure and Re-recording of Information:

[0213] Information was recorded, read out, erased, and re-recorded onthe optical recording medium prepared in Example 3 by use of an opticalsystem shown in FIG. 11. The optical system of FIG. 11 is composed of anargon laser light source 11, a shutter 12, an ND filter 13, a beamexpander 14, a quarter-wave plate 15, Glan-Thomson prisms 16 and 22, abeam splitter 17, a lens 18, a charge coupled device (CCD) 19, anobjective lens 20, an X-Y-Z stage 21, a white light source 23, and acomputer 24.

[0214] A recording medium 25 was set on the X-Y-Z stage as shown andirradiated with laser light having a wavelength of 514.5 nm emitted fromthe argon laser light source 11 for {fraction (1/32)} second per spot,the laser light having been linearly polarized and condensed by theobjective lens 20 to a beam diameter of about 1 μm. Recording wascarried out on the entire surface of the recording medium 25 whilechanging the position with the X-Y-Z stage. After the recordingcompleted, the recording medium 25 was illuminated from its back sidewith white light from the light source 23, and the transmitted light wasdetected by the CCD 19. The image obtained is shown in FIG. 12A. Thewhite spots are the recorded area. It was thus confirmed thatinformation can be recorded and reproduced satisfactorily.

[0215] Then the direction of polarization of the linear polarized lightused above for recording was rotated at 90°. Central nine spots out ofthe white spots of FIG. 12A were irradiated with the polarized light,and the record was detected again in the same manner as described above.The resulting image is shown in FIG. 12B. The central nine spots hadbeen erased, lending confirmation to satisfactory erasure ofinformation.

[0216] The recording medium having information recorded and having partof the recorded information erased was again subjected to recording.Five spots were recorded on the area from which nine spots had beenerased in the same manner as described above. The reproduced image ofthe recording medium is shown in FIG. 12C. Satisfactory re-recording onthe once erased area of the recording medium was thus confirmed.

[0217] While the invention has been described in detail and withreference to specific examples thereof, it will be apparent to oneskilled in the art that various changes and modifications can be madetherein without departing from the spirit and scope thereof.

What is claimed is:
 1. An optical recording method, comprising:constituting a recording layer of a photosensitive material capable ofundergoing a storable and detectable photochemical reaction; setting aninformative object on or above the recording layer at such a positionthat an optical near field generated from the informative object mayreach the recording layer; irradiating at least the area of therecording layer where the informative object is positioned with light tocause the informative object to generate the optical near field; andrecording the distribution of the optical near field on thephotosensitive material as a photoreacting quantity of thephotosensitive material.
 2. The optical recording method according toclaim 1, wherein said informative object is made of a material whichtransmits light for irradiation.
 3. The optical recording methodaccording to claim 1, wherein said informative object has a size of 100nm or smaller.
 4. The optical recording method according to claim 3,wherein said informative object has a size of 25 nm or smaller.
 5. Theoptical recording method according to claim 1, wherein said light forirradiation is laser light.
 6. The optical recording method according toclaim 1, wherein said photosensitive material is at least one of amaterial capable of forming unevenness in accordance with itsphotoreacting quantity to record the distribution of the optical nearfield, a material capable of changing its refractive index in accordancewith its photoreacting quantity to record the distribution of theoptical near field, a material capable of changing its light absorbancein accordance with its photoreacting quantity to record the distributionof the optical near field, a material capable of developing a potentialdifference in accordance with its photoreacting quantity to record thedistribution of the optical near field, and a material containing aphotoreactive component capable of changing its degree of orientation inaccordance with its photoreacting quantity to record the distribution ofthe optical near field.
 7. The optical recording method according toclaim 6, wherein said photosensitive material is a photoreacting polymermaterial capable of forming unevenness in accordance with itsphotoreacting quantity to record the distribution of the optical nearfield.
 8. The optical recording method according to claim 6, whereinsaid photosensitive material is a photorefractive material capable ofchanging its refractive index in accordance with its photoreactingquantity to record the distribution of the optical near field.
 9. Theoptical recording method according to claim 6, wherein saidphotosensitive material is a photoconductive material capable ofdeveloping a potential difference to record the distribution of theoptical near field.
 10. The optical recording method according to claim1, wherein said irradiating is conducted in a state that the informativeobject is set at a position within a 100 nanometers' distance from therecording layer.
 11. The optical recording method according to claim 1,wherein said irradiating is conducted in a state that the informativeobject is set in contact with the recording layer.
 12. The opticalrecording method according to claim 1, wherein said irradiating andrecording are repeated two or more times in accordance with the movementof the informative object.
 13. An optical recording system comprising: arecording layer on or above which an informative object is positionedand which is constituted by a photosensitive material capable ofundergoing a storable and detectable photochemical reaction; and a lightsource capable of irradiating at a time at least the area of therecording layer where the informative object is positioned.
 14. Theoptical recording method according to claim 1, which further comprisesobserving the recorded distribution of the optical near field by anobserving means selected in conformity with the mode of recording. 15.The optical recording method according to claim 14, wherein saidphotosensitive material is at least one of a material capable of formingunevenness in accordance with its photoreacting quantity to record adistribution of the optical near field, a material capable of changingits refractive index in accordance with its photoreacting quantity torecord the distribution of the optical near field, a material capable ofchanging its light absorbance in accordance with its photoreactingquantity to record the distribution of the optical near field, amaterial capable of developing a potential difference in accordance withits photoreacting quantity to record the distribution of the opticalnear field, and a material containing a photoreactive component capableof changing its degree of orientation to record the distribution of theoptical near field.
 16. The optical recording method according to claim15, wherein said photosensitive material is a photoreactive polymermaterial capable of forming unevenness in accordance with itsphotoreacting quantity to record the distribution of the optical nearfield.
 17. The optical recording method according to claim 15, whereinsaid photosensitive material is a photorefractive material capable ofchanging its refractive index in accordance with its photoreactingquantity to record the distribution of the optical near field.
 18. Theoptical recording method according to claim 15, wherein saidphotosensitive material is a photoconductive material capable ofdeveloping a potential difference to record the distribution of theoptical near field.
 19. The optical recording method according to claim14, wherein said irradiating is repeated at a given interval with pulselight to record a changing history of the informative object.
 20. Theoptical recording method according to claim 14, wherein saiddistribution of the optical near field is recorded as a change inrefractive index or absorbance of the photosensitive material, and saidobserving is carried out with a scanning optical near field microscope.21. The optical recording method according to claim 14, wherein saiddistribution of the optical near field is recorded as generation of apotential difference on the photosensitive material, and said observingis carried out with a surface potential microscope.
 22. An opticalrecording medium comprising: at least one recording layer for recordingoptical information with light for irradiation or a generated opticalnear field, wherein the recording layer comprises a polymer materialcontaining a photoreactive component capable of photoisomerization andhaving in the repeating unit thereof at least one group selected fromthe group consisting of a urethane group, a urea group, an amide group,a carboxyl group and a hydroxyl group.
 23. The optical recording mediumaccording to claim 22, which is a recording medium for calculators, anaudio or visual recording medium or a recording medium for recording anoptical near field intensity distribution.
 24. The optical recordingmedium according to claim 22, wherein said optical recording medium hasa single recording layer which is provided on the surface or in theinside of said optical recording medium.
 25. The optical recordingmedium according to claim 22, wherein said optical recording medium hastwo or more recording layers, one of which is provided on the surfacewith the other recording layer or layers in the inside of said opticalrecording medium, or all of which are provided in the inside of saidoptical recording medium.
 26. The optical recording medium according toclaim 22, wherein said optical recording medium has two or morerecording layers with a buffer layer being interposed between tworecording layers.
 27. The optical recording medium according to claim22, wherein said photoreactive component is at least one selected fromthe group consisting of an azo group, a C═C group and a C═N group, allof which are capable of trans-cis photoisomerization.
 28. The opticalrecording medium according to claim 22, wherein said polymer has astructure represented by formula (I), (II), (III) or (IV):


29. The optical recording medium according to claim 22, wherein saidphotoreactive component is contained by 30 to 70% by weight based on thepolymer material.
 30. The optical recording medium according to claim22, which is for holography.
 31. The optical recording medium accordingto claim 22, wherein said photoreactive component is capable ofreversible photoisomerization, the molecular orientation of saidphotoreactive component is controllable by light, and said opticalrecording medium is capable of recording, reading out and erasinginformation.
 32. The optical recording medium according to claim 31,wherein said recording and reading out is carried out with linearlypolarized light, and said erasing is carried out with circularlypolarized light, random polarized light or linearly polarized lightwhose direction of polarization is different from that of the linearlypolarized light used for recording.
 33. An optical recording mediumhaving a recording layer comprising: a photosensitive material capableof undergoing a storable and detectable photochemical reaction, saidrecording layer having recorded a distribution of an optical near fieldgenerated from an informative object being irradiated to furnish atleast one of pieces of information (1) to (4): (1) a record of aninstantaneous form of a moving informative object; (2) a record ofmovement of an informative object which is a fine particle movable byradiant pressure of light; (3) a record of movement of an informativeobject which is an autonomically moving living organism; and (4) arecord of changing history of an informative object which undergoeschange with time that can be recorded as optical information.
 34. Theoptical recording method according to claim 1, wherein said setting isconducted in a state that the informative object is set on the recordinglayer.
 35. The optical recording system according to claim 13, whereinthe informative object is positioned on the recording layer.